1. Field of Invention
The present invention relates to a power converter, and more particularly to a flyback power converter using half-bridge structure.
2. Description of the Related Art
Power converters have been used to convert an unregulated power source to a regulated voltage or a regulated current. Referring to FIG. 1, it illustrates a schematic circuit diagram of a conventional flyback power converter. A transistor 15 controlled by a control signal PWM switches a transformer 50 to transmit the power from the primary-side of the transformer 50 to a secondary circuit and to regulate an output voltage VO. A capacitor 42 and a snubber resistor 41 are used to store and consume energy of a leakage inductor of the transformer 50. The energy of the leakage inductor of the transformer 50 must be completely dissipated; otherwise, a drain voltage of the transistor 15 will be continuously increased until the energy of the leakage inductor is fully dissipated by the snubber resistor 41. The energy therein is given as,
                                          V            R41            2                                R            41                          =                              1            2                    ×                      L            I                    ×                      I            P            2                    ×          f                                    (        1        )            where VR41 is the voltage across the snubber resistor 41; R41 is the resistance of the snubber resistor 41; f is the switching frequency.
One of the drawbacks of the conventional flyback power converter is that the leakage inductor of the transformer 50 consumes considerable power and thus reduces the efficiency of the power converter. Besides, an operating voltage VDS of the transistor 15 is increased because of the energy involving the leakage inductance, to which the voltage VR41 adds the operating stress of the transistor 15 (VDS=VIN+VR41). Another drawback of the conventional flyback power converter is a NTC (negative temperature coefficient) thermistor 11 that is disposed for to restricting an inrush current of the flyback power converter. When the flyback power converter is turned on, an inrush current is induced when charging the capacitor 18. The inrush current reduces reliability of the flyback power converter. In addition, the NTC thermistor 11 increases the power consumption and operating temperature of the flyback power converter.
Therefore, it is desirable to improve the efficiency of the flyback power converter by eliminating the necessity of the thermistor and reducing the power consumption by the leakage inductor of the transformer. Furthermore, it is also desirable to reduce the operating stress of the power converter for higher reliability.